Well, the prime suspects we know about would be either Sidious or Ventress, could be Maul as well but I doubt he could pass as a jedi
Could also be someone we haven't seen before, but I doubt they would bring in someone unknown
Who knows? They might. I highly doubt it was Maul given he is likely still angry at Sidious for killing his brother known as Savage I believe to do something such as that possibly. I doubt it was Ventress because Ventress doesn't do sneaky things like that, she attacks without stealth, period. Sidious might be it though because when you talk about the title, its "The Jedi who knew too much," so it could be that Letta was going to tell Ahsoka it was the Chancellor, who is known as Darth Sidious with his sith lord name, therefore exposing the Chancellor in a way.
But still, theres not enough proof to make actual statements of it being whoever, but I do have suspicions of either a new person being introduced, or it being sidious.